Damage to crops by predators (i.e., insects, herbivores, and pathogens, including fungi, bacteria, and viruses), results in substantial annual losses in agricultural production. Man has created and employed a wide range of chemicals in attempting to reduce damage to plant crops. Many environmental problems have been created by the widespread use of chemicals that may only provide a transient level of protection for crops. Chemicals also suffer from the disadvantage that all organisms in an area may be indiscriminately treated, causing needless damage to many beneficial organisms. Many chemicals are also potentially toxic to man and animals.
Attempts to reduce crop damage have included selective breeding for resistance, but resistance traits can frequently be controlled by many genes making it difficult (or impossible) to genetically select a desired attribute. Decreased crop yields are also commonly encountered in resistance strains. Accordingly, there exists a strong need for compositions and processes to improve the resistance of plants under attack by herbivores.
Plants have evolved inducible defensive mechanisms that respond to attacks by predators (C. A. Ryan, 1990, Ann. Rev. Phytopathol. 28:425; D. J. Bowles, 1990, Ann. Rev. Biochem, 59:873; M. Chessin and A. E. Zipf, 1990, The Botanical Review 56:193; D. L. Dreyer and B. C. Campbell, 1987, Plant, Cell and Environ. 10:353). One mechanism involves systemic synthesis of serine proteinase inhibitors that are accumulated at distal tissue sites in plants. The proteinases can inhibit the digestive enzymes of insects and microorganisms (T. R. Green and C. A. Ryan, 1972, Science 175:776; C. A. Ryan, 1978, TIBS 3(7):148; V. A. Hilder, A. M. R. Gatehouse, S. E. Sheerman, R. F. Barker, D. Boulter, 1987 Nature 330:160; R. Johnson, J. Narvaez, G. An, C. A. Ryan, 1989, Proc. Natl. Acad. Sci. U.S.A. 86:9871). Proteinase inhibitors can be detrimental to the growth and development of insects from a variety of genera including Heliothis, Spodoptera, Diabiotica and Tribolium (Ryan, supra; Broadway, supra; Rechsteiner, supra). Several families of polypeptides have been described that inhibit serine proteinases, including: the Kunitz family (e.g., Soybean trypsin inhibitor); the Bowman-Birk family; (e.g., Soybean proteinase inhibitor); the Potato I and Potato II families; the Barley trypsin inhibitor family; and, the Squash inhibitor family.
Wounding of plants by animals, including insects, and pathogens or mechanical damage, reportedly induces transcriptional activation of proteinase inhibitor genes and protein synthesis (J. S. Graham, G. Hall, G. Pearce, C. A. Ryan, 1986, Planta 169:399). The latter wound-response has reportedly been described in a variety of species including; tomato (J. S. Graham, G. Pearce, J. Merryweather, K. Titani, L. Ericsson, C. A. Ryan, 1985, J. Biol. Chem. 260(11):6555; J. S. Graham, G. Pearce, J. Merryweather, K. Titani, L. H. Ericsson, C. A. Ryan, 1985, J. Biol. Chem. 260(11):6561), potato (C. A. Ryan, 1968, Plant Physiol. 43:1880), alfalfa (W. E. Brown and C. A. Ryan, 1984, Biochemistry 23:3418; W. E. Brown, K. Takio, K. Titani, C. A. Ryan, 1985, Biochemistry 24:2105), cucurbits (D. Roby, A. Toppan, M. T. Esquerre-Tugaye, 1987, Physiol. Mol. Pl. Pathol. 30:6453) and poplar trees (H. D. Bradshaw, J. B. Hoflick, T. J. Parsons, H. R. G. Clarke, 1989, Plant Mol. Biol. 14:51). Wounding reportedly results in the rapid accumulation of proteinase inhibitors not only in wounded leaves but also in distal, unwounded leaves, suggesting that a signal, or signals, released from the wound site travels throughout the plant. Transport of these signals is mediated locally through intercellular and intracellular fluids that permeate wound or infection sites (Green, T. R. and C. A. Ryan, Science 175:776-777, 1972) or travel systemically through the vascular system of plants (Kuc, J. and C. Presisig, Mycologia 76:767-784, 1984: M. Kopp, et al., Plant Physiol. 90:208-216, 1990; and K. E. Hammond-Kosack, et al., Physiol. Mol Plant Path. 35:495-506, 1989). Proposed wound signals include: pectic fragments derived from the plant cell wall (C. A. Ryan and E. E. Fanner, 1991, Annu. Rev. Plant. Physiol. Mol. Bio. 42:651); the lipid-derived molecule, jasmonic acid (E. E. Farmer and C. A. Ryan, 1990, Proc. Natl. Acad. Sci. U.S.A. 87:7713); the plant hormone, abscisic acid (H. Pena-Cortes, J. J. Sanchez-Serrano, R. Mertens, L. Willmitzer, S. Prat, 1989, Proc. Natl. Acad. Sci. U.S.A. 86:9851); electrical potentials (E. Davies, 1987, Plant, Cell and Environ. 10:623; J. F. Thain, H. M. Doherty, D. J. Bowles, D. C. Wildon, 1990, Plant, Cell and Environ. 13:569); and, more recently, an 18-amino acid polypeptide called systemin (G. Pearce, D. Strydom, S. Johnson, C. A. Ryan, 1991, Science 253:895).